I've noticed a curious problem with the latest monthly release of eBooks from Baen in Mobi format.

Baen (annoyingly) always deliver books with the author metadata set to, for example, "Robert A. Heinlein" rather than "Heinlein, Robert A.", so the first thing I always do after adding the book to Calibre is to edit the metadata and change both the "author" and "author sort" fields to the "surname, forename" format which I prefer, and then send the book to my Kindle PW.

The problem is that, for all the books released in this month's Baen monthly subscription package, the resulting books are sorting on the Kindle by the author's first name, not their surname. Eg, "The Man Who Sold the Moon" by Robert A. Heinlein is sorting under "Robert" in author sort mode, not under "Heinlein". I've deleted the book from the Kindle, re-edited the metadata in Calibre to make sure that I really had changed it, and re-sent the book to the Kindle. No difference.

If I convert the book from Mobi to AZW3 format, and then re-send it to the Kindle, the book sorts correctly under "Heinlein", so it looks as if Calibre is not correctly setting the metadata for the Mobi version of the book.

No, it's just not something I have ever found the time/interest to implement.

The MOBI metadata code was not originally written by me, which makes modifying it to deal with joint mobi files painful. And then MOBI is a reverse engineered format, which means that every time I make a change in some aspect of MOBI handling the only way I can validate that change is by creating lots of test files and uploading them to a Kindle and seeing what it does.

I understand. Thanks for explaining. Might I make a small request? Would it be possible to indicate in Calibre's format description in some way whether or not a Mobi file is a joint Mobi file or a pure MOBI6 file? I don't know of any way to find out which it is.

If you open it in the calibre viewer, the titlebar will show you [KF8] which means that the file is either a joint file or a pure KF8 file with a .mobi extension (the latter is almost never seen), so it is likely a joint file.